JUNE1966
Terpenoids.
PENTACYCLIC TRITERPENE HYDROCARBOXS
LVII.'
1945
Mass Spectral and Nuclear Magnetic Resonance Studies of Pentacyclic Triterpene Hydrocarbons2 JERRY KARLINER AND CARLDJERASSI Department of Chemistry, Stanford University, Stanford, California Received January 11, 1966
As an extension of earlier ~ t u d i e s the , ~ mass spectra of A12-oleanene,Al2-ursene, and deuterated analogs were determined. The use of deuterated derivatives served to confirm some fragmentation modes postulated earlier and caused revision of others. The intense and highly characteristic peak a t m/e 203 was shown to be a composite fragment ion and its genesis is discussed. The synthesis of deuterated derivatives is described and suggested mechanisms for the formation of fragment ions are given. With the aid of high-resolution nmr spectroscopy and the use of deuterated derivatives, it was possible to identify all of the methyl signals in the nmr spectrum of ii12-oleanene.
A detailed study of the mass spectral fragmentation patterns of pentacyclic triterpenes by noting peak shifts in various derivatives has already been de~cribed.3,~ This has led to important generalizat'ions, particularly in A12 compounds, and has been extremely useful in structural elucidation studies of AI2-oleanene and A12ursene derivatives.4,5 As described earlierj3 the dominant process upon electron impact of t'riterpenes cont'aining a double bond at C-12 results from abstraction of an electron from the olefinic ?r bond, followed by a retro-DielsAlder type6 opening of ring C. The retro-Diels-Alder fragment then undergoes further decomposition reactions, and it is toward these subsequent fragmentations that the main emphasis of the present studies are directed. Precise location of a. fragment ion can be determined by deuterium labeling as well as substituent labeling. However, it is only with the aid of isotope labeling that hydrogen-transfer react'ions can be detected. Furthermore, the exact nature of the m/e 203 fragment ion (aide infra) necessitated investigating the mass spectra of deuterated derivatives. It is for these reasons that a program was initiated to prepare deuterium-labeled derivatives of A12-oleanene and AI2ursene. Since some of these compounds were to be deuterated at the tert'iary methyl groups (of which there are eight), it was also decided to investigate the nuclear magnetic resonance spectra of A12-oleaneneand A12-ursene. Synthetic Studies The synthetic transformations used for the preparation of deut>erium-labeled compounds in the (1) For paper LVI, see 0. Kennard, L. R . di Sanseverino, H. Vorbriiggen, and C . Djerassi, Tetrahedron Letters, 3433 (1965). The present article should also be considered part CV in the series, "Mass Spectrometry in Structural and Stereochemical Problems." (2) Financial assistance (Grant No. Ghl-06840) from the National Institutes of Health of the U. S.F'ublic Health Service is gratefully acknowledged. (3) C . Djerassi, II. Budzikiewicz, and J. hl. Wilson, Tetrahedron Letters. 263 (1962); H . Budzikiex-icz, ,J. h1. TVilson, and C . Djerassi, J . A m . Chem. Soc., 86, 3688 (1963). (4) J. S.Shannon, Australian J . Chem., 16, 683 (1963). (5) Inter alia, B. 'rursch, r't al., J . Oro. Chem., 28, 2390 (1963); S. Huneck and G. Snatzke, Chem. Ber., 98, 120 (1965); R. 0. Donchaf and J. B. Thomson, Tetrahedron Lettms, 2223 (1965); J. B. Thomson, ibid., 2229 (1965); M . Alauddin, T . A. Bryce, E . Clayton, At. Martin-Smith and G . Subramanian, J . Chem. Soc., 4611 (1965); L. H. A. Elgamal, M. B. E. Fayez, and G. Snatzke, Tetrahedron, 21, 2109 (1965); I. P. Varshney, et al., Tetrahedron Letters, 118i (1965); R . Teschesche and G . Wulff, ibid., 1569 (1965); S. Nakamura, et a!., ibid., 2017 (1965); B. Tursch, J. Leclerq, and G. Chiurdoglu, i b i d . , 41.61 (1965) ; R . E . Taylor-Smith, Tetrahedron, 21, 3721 (1965). (6) For a discussion and other references of electron-impact induced retroDiels-Alder reactions, see H. Budzikiewicz, J. I. Brauman, and C . Djerassi, i b i d . , 21, 1855 (1965).
p-amyrin series are outlined in Chart I. Oleanolic acid (I) was oxidized by the Jones procedure7 to oleanonic acid (11)and the latter was converted by a modified Huang-Minlon reactions to oleanenic acid (111). Lithium aluminum hydride reduction of 111 afforded the primary alcohol IV, which was transformed to the aldehyde V by oxidation with pyridinium trifluoroacetate and dicyclohexylcarbodiimide in dimethyl sulfoxide according to the method of Pfitzner and Moff Upon treatment with ethanedithiol, A12-oleanen-28-al (V) afforded the dithioacetal VI, which led to the desired A12-oleanene-28,28-dz(VII) upon desulfurization with deuterium-containing Raney nickel. lo Methyl acetylglycyrrhetate" (VIII) served as starting material for the preparation of the A12-oleanene derivative isotopically labeled a t C-30. Hydrogenation of VI11 with platinum oxide in acetic acid furnished the 11-deoxy derivative IX,12 which upon saponification afforded the hydroxy acid X. Chromic acid oxidation7 followed by Huang-JIinlon reduct.ions gave A12-oleanene-30-carboxylicacid (XII). The same procedure (XI1 + XI11 + XIV + XV + XVI) was employed for converting XI1 t'o the desired A1*-oleanene-30,30-dz (XVI) as has already been described above for similar transformations at C-28. p-Amyrin (XVII) was used for the preparation of analogs of A12-oleanene (XXI) deuterated in rings A and C. Jones oxidation' of XVII furnished A12-oleanen-3-one (XVIII) and the latter was treated with p-toluenesulfonylhydrazide in methanol-dioxane to afford XIX. The tosylhydrazone mas not isolated but was reduced directly to A12-oleanene-3,3-d2(XX) by means of lithium aluminum deuteride, using deuterium oxide in the work-up pr0cedure.1~ A12-01eanene (XXI) was prepared from XVIII by the HuangllIinlon procedures and oxidized to the a,&unsaturated ketone XXII by heat'ing under reflux with chromium trioxide in acetic acid.l4 As shown previously,12 (7) K. Bowden, I . M. Heilbron, E . R. H. Jones, and B. C . L. Weedon, J . Chem. Soc., 39 (1946). (8) Huang-Minlon, J . A m . Chem. SOC.,68, 2847 (1946). (9) K. E. Pfitzner and J. G . Moffat, ibid., 86, 3027 (1963). (10) D. H. Williams, J. M. Wilson, H. Budzikiewicz, and C . Djersssi, ibid., 86, 2091 (1963). (11) L. Ruzicka and H. Leuenberger, Nelu. Chim. Acta, 19, 1402 (1936); L. Ruzicka, M . Furter, and H. Leuenberger, ibid., 20, 312 (1937); C. Djerassi and C. h l . Foltz, J . Am. Chem. SOC.,76, 4085 (1954). (12) L. Ruzicka, H. Leuenberger, and H. Schellenberg, H e h . Chim. Acta, 20, 1271 (1937). (13) hf. Fischer, Z. Pelah, D. H. Williams, and C. Djerassi, Chem. BeT., B8, 3236 (1965). (14) R. Budziarek, W. hlanson, and F. 9. Spring, J . Chem. Soc., 3336 (1951).
KARLINER AND DJERASSI
1946
VOL.31
&
CHARTI
28
H
R. ,, 23 24
I, R = C HOH
IV, R- CHzOH V, R=CHO
11, R = O I I I , R = C HH
VI, R-CH;;] VI11 R=CHD,
COOCH;
Ad H
IX,R=
185). Since the mass spectra of A12-oleanene (Figure 2) and A12-ursene (Figure 3) are qualitatively the same, the peak migrations and mechanistic genesis of fragment ions other than m/e 203 of these two classes will be considered together. The individual peaks will now be discussed in detail. Peak M - 153 (m/e 257 in Figures 2 and 3).-This peak represents the most abundant fragment ion above m/e 218. Inspection of Table I reveals that a major part of this peak is shifted to m / e 259 in compounds VII, XVI, and XXXVI, and is displaced entirely to m/e 259 in the 11,ll-dideuterated substance XXIII. This
JUNE1966
PENTACYCLIC TRITERPENE HYDROCARBONS
1949
E
f
0 mA
U
mh
Figure 2.-Mass spectrum of AX2-oleanene(XXI). Figure 3.-Mass spectrum of A12-ursene (XLI).
4
b
a
Peak M - 192 (m/e 218 in Figures 2 and 3).The intense and diagnostically important m/e peak (base peak in XXI and XLI) has been discussed in considerable detail elsewhere.394,6 It is sufficient to note here that the earlier mechanistic interpretation for the formation of the m/e 218 species by a retro-DielsAlder decomposition ( i e . , a --t d)3z4v6is consistent with the spectra of all the deuterated derivatives examined in this study. For the sake of brevity only one of the possible resonance forms of d is shown.
c, m l e 257
R=CH,; R'==H or R = H ; R ~ = C H ~ a
implicates rings C, D, and E for the charge-containing portion, consistent with the observation that the mass spectrum of the 3,3-dz analog XX exhibits this peak a t m/e 257. Furthermore, the mass spectrum of XLV demonstrates that C-27 is also retained in this fragment. A mechaniam consistent with these data involves homolylic cleavagez4of the 9-10 bond in the molecular ion a8p4 to afford b, followed by hydrogen transfer from (3-26 to C-7 with concomitant homolysis of the 7-8 bond to afford the resonance stabilized species c (m/e 257) above. (24) A fishhook ( N ) is used to designate the shift of a single electron, : while the movement of an electron pair is signified by an arrow ( N ) H. Budzikiewicz, C. Djeraesi, and D. H. Williams, "Structure Elucidation of Natural Products by Mass Spectrometry." Vol. 11, Holden-Day, Ino., Sen Francisco Cslif., 1964, pp 1-3.
d, mle 218
R-CH,; R'=H or R = H ; R f = C H 2
Peak M - 207 (m/e 203 in Figures 2 and 3).-The fragment ion appearing at m/e 203 in the mass spectra of A12-oleanene (XXI) and A12-ursene (XLI) proved to be the most interesting peak from a mechanistic standpoint and in fact was the chief reason why the present deuterium-labeling studies were undertaken. This
1950
KARLINER
AND
moiety results from further loss of 15 mass units from the retro-Diels--Alder fragment d. The mass spectrum of A12-oleanene-28,28-d2 (VII) was investigated in order to determine whether the C-17 substituent was involved in the m/e 218 + m/e 203 fragmentation and it was found that VI1 accounted for only ca. one-third loss of the C-28 methyl group. Similarly the mass spectrum of AI 2-oleanene-30,30-d2(XVI) exhibited ca. one-third loss of the axial C-20 substituent and, therefore, it is reasonable to assume that the remaining one-third loss of 15 mass units results from expulsion of the C-29 methyl group. It was thought that perhaps loss of the methyl groups from C-20 was facilitated by a ring-closure reaction, as shown in d + e. An
e, m/e 203
ti
examination of Dreiding models indicates that such ring formation is feasible. However, the molecular models suggest that in the 18a series, bond formation between C-11 and C-20 is not possible. Therefore, in order to determine whether ion e was an important contributor to the m/e 203 fragment, 18cu-A12-oleanene28,28-d2 (XXIX) was prepared and its mass spectrum determined. If path d 3 e were operating in the electron impact induced decomposition of A12-oleanene, then, since such a pathway is not possible in the D/E trans-fused compound XXIX, the latter would be expected to lose the C-28 methyl group to a greater extent than observed for Al2-oleanene-28,28-d2 (VII). However, it wa,s found that in the mass spectrum of X X I X expulsion of the (3-28 methyl was responsible for 35% of the m/e 218 + m/e 203 fragmentation. This figure coincides with that noted in the loss of the C-28 methyl grouping in VI1 and forces one to conclude that d 4e does not represent a significant pathway for the genesis of the m/e 203 species. The finding that the m/e 203 results from the retroDiels-Alder fragment d by equal loss of the methyl substituents at C-17 or C-20 can be explained in the following manner. Fragment d may suffer a loss of the C-28 meth,yl group, while undergoing homolytic scission of the 15-16 bond to afford the resonance stabilized dienyl cation f. Alternatively, d, upon expulsion of a methyl group bonded to C-20 (either the C-29 or the C-30 methyl group) with concomitant homolytic cleavage of the 18-19 bond results in another
d
a
f, m/e 203
g, m/e 203
DJERASSI
VOL.31
dienyl cation g. I n A12-oleaneneand A12-ursene derivatives, oxygen-containing substituents a t C-17 or C-20 (COOCH,, CHZOH, CH20Ac, or CHO) are lost in preference to methyl from the retro-Diels-Alder fragment ion owing to stabilization of the expelled radical by oxygen. The mechanistic interpretation of the origin of the m/e 203 species in the oleanene series would lead one to predict, a priori, that in the ursene series the formation of the m/e 203 moiety would result from 50% loss of the C-28 methyl radical and 50% expulsion of the C-30 methyl radical, since A12-ursene contains only a single methyl group a t C-20. An examination of the mass spectrum of A12-ursene-28,28-d2(see Table I) shows that, in fact, cleavage of the C-28 methyl group accounts for only 35% of the formation of the ?n/e 203 peak. It, therefore, appears that in A12-ursene, as had been noted in A12-01eanene,the loss of 15 mass units, upon generation of the nz/e 203 fragment from the retro-Diels-Alder species d, involves all three ring-E methyl groups to the same extent. These considerations lead to three alternative fragmentation modes for the formation of the m/e 203 cleavage product. Paths d -+ h and d 3 i are analogous to those discussed for the similar decomposition of AI2-oleanene (see f and g) while route d + j involves expulsion of the C-29 methyl radical and homolytic fissiion of the 17-18 and 15-16 bonds.
d
h,m/e 203
r:
i, m/e 203
d
15
d
j,
Ne
203
Peak M - 219 (m/e 191 in Figures 2 and 3).-The results shown in Table I are in full agreement with previous studies on the origin of this peak.3 Thus compound XX implicates ring A as being part of this fragment, while VII, XVI, and XXXVI indicate that a major portion of this peak does not involve rings D and E. Furthermore, the spectrum of XXIII reveals the absence of C-11 as part of the m/e 191 fragment and XLV shows that C-27 is likewise not involved. The mechanism previously proposed3 for this species involved heterolytic fission of the allylically activated 8-14 bond to form k,consisting of a tertiary carbonium ion and an allylic radical. Hydrogen transfer from (3-26 to C-11 with simultaneous bond breaking a t 9-11 would then result in the formation of 1, m / e 191. Although the results described in the present studies can
JUNE1966
PENTACYCLIC TRITERPENE HYDROCARBONS
1951
12
n, mle 189 R-CH3; R’=H or R-H:;Rr-CH3
k R‘
1, m/e 191
It seems appropriate a t this point to suggest a mechanistic modification in the characteristic mass spectral cleavage reaction of 12-oxygenated triterpenes. Recently a comparison has been made% of A9(”)-12ketones in steroids and triterpenes and this does not need to be considered further. However, saturated 12-ketones in triterpenes, such as 3~-acetoxy-12-oxoursane (XLVI) exhibit a base peak at m/e 234 which
be reconciled with such a mechanism, hydrogen transfer from C-7 would be preferred to a similar transfer from C-26 (transfer of a secondary us. a primary hydrogen atom) and it appears that k + m is energetically more favorable than k + L as a representation of the generation of the m/e 191 fragment. XLVI
was shown to be the fragment indicated schematically in formula XLVI. The formation of this m/e 234 fragment may be considered as occurring by a decomposition mode which is directly analogous to the generation of the base peak of 7-keto steroids.n Thus, cleavage of the 8-14 bond with expulsion of an electron results in the molecular ion 0,which may then undergo homolysis of the 9-11 bond to furnish the resonance stabilized radical ion p (m/e 234).
m, mfe 191 R-CH3; R‘=H or R-H; R‘-CH3 Peak M - 221 (m/e 189 in Figures 2 and 3).-The fragment occurring a t m/e 189 has been found to be more complicated than thought earlier.* An examination of Table I reveals that the predominant composition of this peak involves the right-hand portion of the triterpene molecule. Thus, compoundsVI1 and XXXVI indicate that C-28 is retained in this species, while compound XVI implicates C-30 as part of the fragment ion. A major part of the peak remains a t m/e 189 in XX and XXIII demonstrating that ring A and C-11 are not involved in the m/e 189 species. A striking entry in Table I is that of compound XLV. The pronounced shift to m/e 190 in this compound indicates that the C-27 methyl group is lost. (The alternative explanation that a hydrogen atom from the vinylic C-12 position is expelled is unlikely.) These results lead to structure n, a stable cyclopropenium cation, as a representation of the m/e 189 fragment, although its manner of formation is not obvious. (25) H. Budaikiewicz, C. Djerassi, and D. H. William, “Interpretation of Mass Spectra of Organic Compounds,” Holden-Day Inc., San E’ranoiaco, Calif., 1964, p 11.
0
p, m/e 234
Discussion of Nuclear Magnetic Resonance Spectra An early investigation of the nuclear magnetic resonance spectra of AI2-oleaneneand AI2-ursenederivatives at 40 Mc revealed overlapping signals in the methyl region and specific assignments to most of the methyl groups could not be made.% Other reports involving nuclear magnetic resonance studies of pentacyclic triterpenes have been confined to lupane,2Q (26) L. Takes and C. Djeraesi, Sleroida, 6, 493 (1965). (27) Beugelmana, R. H. ShapirO, L. J. Durham, D. H. Willisma, H. Budzikiewica, and C. Djerassi, J . A m . Chem. Soc., 86, 2832 (1964). (28) M. Shamma, R. E. Glick, and E. Glotter, J . Orp. Chem., 47, 4512 (1962). (29) J. M. Lehn and G. Ourisaon, B d l . SOC.Chtm., Fronce. 1137 (1982).
R.
KARLINER AND DJERASSI
1952
VOL.31
0.87 I
f
6
Figure 4.-Partial 100LMc nuclear magnetic resonance spectrum (high-field region) of A12-oleanene(XXI). Figure 5.-Partial 100-Mc nuclear magnetic resonance spectrum (high-field region) of A1*-01eanene-28,2S-&(VII).
hopane,w betulin,al and Al2-01eanenea2derivatives. The investigations involved determining the spectra of compounds differing in the nature or placement of a functional group and examining the shift of methyl peaks in such derivatives. I n view of the availability of deuteriated derivatives of the AI2-oleanene and Al2-ursene hydrocarbons, we decided to investigate the nmr spectra of these compounds. The nmr spectrum of A12-ursene (XLI) was quite complex (even at 100 Mc) owing to secondorder perturbation effect@ exhibited by the two secondary methyl groups and was not examined further. However, the methyl region (6 0.0-1.2) in the nmr spectrum of A12-oleanene (XXI) was readily discernible and forms the basis of the present discussion. The high-field region of the 100-Mc nmr spectra of A12-oleanene ( X I ) , A12-oleanene-28,28-d2 (VII), Al2(30) 5. Huneck and J. M. Lehn, Bull. SOC.Chim., France, 1702 (1963). (31) J. M. Lehn and A. Vystrfil, Tetrahedron, 19, 1733 (1963). (32) B. Tursch, R. Savoir, and G. Chiurdoglu, BdE. ,5400. Chim. Bduea 75, 107 (1966). We wish to thank Dr. Turach for supplying us with a preprint of thin paper. (33) N. 6.Bhacca and D. H . Williams, “Applications of NMR SpectrosCOPY in Organic Chemietry,” Holden-Day, Inc., San Francisco, Calif., 1964, p 35.
d Figure &-Partial 100-Mc nuclear magnetic resonance spectrum (high-field region) of ALZ-oleanene-30,30-& (XVI ). Figure 7.-Partial LOO-Mc nuclear magnetic resonance spectrum (high-field region) of A%leanen-3091 (XIII).
oleanene-30,30-d2 (XVI), and A12-oleanen-30-ol (XIII) are shown in Figures 4-7, respectively. The parent substance XXI exhibits peaks corresponding to one methyl group at 6 1.13, 0.97, and 0.93, a signal a t 6 0.87 due to three methyl groups, and a peak a t 6 0.83 corresponding to two methyl groups. The spectrum of VI1 shows a marked decrease in the resonance a t 8 0.83, thereby ascribing one of the methyl groups a t this frequency to C-28 in XXI. Similarly, the area at 6 0.87 in Figure 4 diminishes in the spectrum of A12oleanene-30,30-d2 (XVI), demonstrating that the C-30 methyl signal in Al2-o1eanene (XXI) appears a t 6 0.87. That the C-29 methyl resonance also occurs a t 6 0.87 in the nmr spectrum of XXI can be seen by comparing the spectra of A12-oleanene(Figure 4) with that of its 30-hydroxy derivative (XIII) (Figure 7). Of the three methyl resonances present a t 6 0.87 in the parent hydrocarbon XXI, two methyl signals are deleted from this value and a new methyl peak appears a t 6 0.89 in the spectrum of XIII. One of the methyl signals removed is obviously the one ascribed to C-30 (no such methyl group is present in XIII), while the (3-29 methyl group has shifted to 6 0.89 owing to the
PENTACYCLIC TRITERPENE HYDROCARBONS
JUNE1966
small deshielding eflect of the y-hydroxyl group. The remaining methyl resonance at 6 0.87 in Figure 4 is attributed to the C-23 methyl group since the latter has an environment similar to the methyl group a t C-29 and is not associated with any unusual interactions which may give rise to shielding or deshielding phenomena. The remaining methyl groups are involved in 1,3diaxial interactions and will be considered together. It has previously been shown that axial methyl groups, upon introduction of an axial @-methyl group are deshielded by 0.05-0.10 ppm.= The methyl signal a t lowest field (s 1.13) in Figure 4 is undoubtedly due to the methyl protons at C-27. This observed deshielding is a consequence of the (3-27 methyl group being homoallylic and suffering a l,&diaxial interaction with the 18-19 bond. Of the methyl groups which have not been accounted for, the C-24 and C-26 suffer one diaxial interactilon (with C-25) while the C-25 methyl group is associated with two diaxial interactions (C-24 and C-26). This leads to the conclusion that the C-25 methyl group occurs at higher frequency than the other two methyl substituents and is accordingly ascribed to the resonance at 6 0.97. These considerations would also predict that the signals due to the C-24 and C-26 methyl protons would occur at the same frequency, as is obviously not the case. This apparent discrepancy can be explained upon examination of a Dreiding model of the A12-oleanene molecule. The nonrigidity of ring C permits distortion of this molecule to occur. If distortion were to occur, in order to alleviate any of the l,&diaxial interactions present in XXI (C-24 us. C-25; C-25 us. C-26; C-27 us. C-lS), the C-26 methyl group would be forced into the region above the plane of the olefinic bond, a situation known to give rise to shielding effects." On this basis, one of the methyl groups responsible for the peak at 0.83 is assigned to C-26, while the resonance at 6 0.93 is ascribed to the C-24 methyl group. To summarize, the assignments of methyl groups in the high-field region in the nmr spectrum of A12-01eanene (XXI) are 6 value methylgroup
1.13 27
0.97 0.93 25 24
0.87 23,29,30
0.83 26,28
Experimental Sections? Method of Calculating Peak Shifts.-The generally low isotopic purity of products obtained by deuterium-containing Raney (34) L. M. Jackman, "Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," Pergamon Press Ltd., London, 1959, p 53. (35) G. Slomp, Jr., and B. R. McGarvey, J . Am. Chem. SOC.,81, 2200 (1959). (36) See ref 34, p 129. (37) The mass spectra were determined with E Consolidated Electrodynamics Corporation mass apectrometer, No. 21-103C, equipped with a direct inlet system. The ionizing current was kept at 50 pa, while the ioniring energy was maintained at, 70 ev and the source temperature at 250°. The optical rotatory dispersion measurements were made on a Japan Spectroscopic Co. (Jasco) automatically recording spectropolarimeter Model ORD-5. Melting points wen) determined on a Kofler hot stage and are uncorrected. Analytical thin layer chromatoplates with a thickness of 0.25 mm of silica gel G (E. Merck A.G., Darmstadt) were used and the spots were detected by spraying with a 2Yo ceric sulfate solution in 2 N sulfuric acid. Preparative thin layer chromatoplates of 0.50-mm thickness were prepared from a slurry of silica gel H (E. Merck A. G., Darmstadt) in 5% silver nitrate solution and compounds were detected by viewing in a direct path of light. Nuclear magnetic resonance spectra were determined with Varian Associates Model A-60 and HR-100 nmr spectrometers using tetrsmethylsilane as an internal reference standard. We wish to express our appreci-
1953
nickel desulfurizationlo necessitated the judicious calculation of peak shifts. An example of such a calculation for the m/e 203 fragment in A1%leanene-30,30-dz (XVI) is presented in Table 11. Compound XVI contained 7% do ( m / e 410), 19% dl ( m / e 411), 62% d2 ( m / e 412), and 12% d3 ( m / e 413). Analysis of the mass spectrum of the unlabeled compound X X I indicated the relative abundance of fragments shown in Table I1 for the m/e 201-mle 208 region, after correction for C1* content. The anticipated spectrum of XVI in this region, assuming complete retention of the isotope label, was then calculated by the following procedure. Of the total 7% do species, 6% (79 X 7%, to the nearest integer) would be expected to appear at m/e 203, while 1% (10 X 7%) would result at m/e 205. Similarly, contributions of 15% (79 X 19%), 1% (5 X 19%), and 2% (10 X 19%) a t m/e 204, 205 and 206, respectively, would be expected from the dl species. Performing the calculations for the 62% dz and 12% da species and addition of the contributions to a given m/e value results in the predicted spectrum. Thus, if no loss of the C-30 methyl group had occurred, 51oJ, of the peak group discussed should occur a t m/e 205 and 7% at m/e 203; Le., 88% (51/58 x 100) of the total intensity of these two peaks should appear at m/e 205 and 12% (7/58 X 100) at m/e 203. Of the total intensity a t m/e 205 and m/e 203 in the observed spectrum, 50.2% (88% X 57) would be expected a t m/e 205 and 6.8% (12% x 57) at m/e 203 if the (2-30 methyl group were not expelled. The deviation from these values indicate that 35% (50.2 - 33/50.2) X 100 = 33.5% to the nearest 5%) of the C-30 methyl group is lost in compound XVI.
TABLE I1 IONCURRENT(yo)OF m/e 201 TO mle 208 REGION Compd
A12-01eanene (XXI) A'l-oleanene30,30-d2 (XVI)
201
202
203
2
2
79
m/i 204 205
5
10
. . . . . . 6 ... . . . . . . . . . 15
1 1 49
......
1
......
7 24
1
............... Predicted Obsd
3
3
16 10
51 33
206
2
207
208
......
......... 2 3 9 14 12
...... 6 1 7 11
1 1 2 4
A12-01eanene-28-carboxylicAcid (111).-Jones reagent' was added dropwise to a stirred solution of 690 mg of crude oleanolic acid (I) in acetone until an orange color persisted for 1 min. The mixture was then concentrated to one-half its volume, diluted with water, and extracted twice with ether. The combined ether extracts were washed twice with water and dried over anhydrous magnesium sulfate. Removal of the solvent under reduced pressure gave an oil which crystallized upon addition of a few drops of methanol. Recrystallization from methanol afforded a material (11, 440 mg) melting a t 154-158', which was not purified further but subjected directly to a modified Huang-Minlon reduction .a The oxidation product I1 and 5 ml of 95% hydrazine in 20 ml of diethylene glycol and 15 ml of absolute ethanol were heated under reflux for 1 hr under nitrogen. After adding 2.1 g of potassium hydroxide pellets and heating under reflux for an additional 0.5 hr, the condenser was removed and the reaction mixture was warmed slowly until an internal temperature of 195' was attained. The condenser was returned and the reaction mixture was maintained at 195-205" for 4 hr while being kept under an atmosphere of nitrogen. After cooling to room temperature, the solution was poured into dilute hydrochloric acid, extracted with ether, and the ethereal extract was washed twice with water. Drying over anhydrous magnesium sulfate, followed by removal of soIvent under diminished pressure, afforded 420 mg of product (111): mp 264-266' after recrystallization from methanol, [01]'.80 +45.56 (c 0.72). Anal. Calcd for CaoHdsO~: mol wt, 440. Found by mass spectrometry: mol wt, 440. A1~Oleanen-28-ol(N).-To a stirred suspension of 110 mg of lithium aluminum hydride in anhydrous ether was added dropation to Mr. J. Smith for mass spectral determinations, to Dr. L. J. Durham for obtaining the nmr spectra, and to Messrs. E. Meier and J. Consul of the Stanford University Microanalytical Laboratory for microanalysis determinations.
1954
KARLINER AND DJERASSI
VOL.31
wise a solution of 205 mg of AlZ+leanenic acid (111) in anhydrous Pfitzner andMoffat9 aa described for the similar preparation of ether and the resulting suspension maintained at reflux overnight A1%leanen-28-al (V). This method produced 28 mg of ~ 1 2 while under nitrogen. While cooling in an ice-water bath, water oleanen-30-a1 (XIV): mp 129-133' after a single recrystallizawas added dropwise to decompose excess hydride reagent. Dilute 3.69 fi (w), 5.80 p (e); tion from methanol-chloroform; ": : :A hydrochloric acid was added to dissolve the inorganic hydroxides [ C Y ] ~ . ~+110.2' D ( c 0.49). Anal. Calcd for CJ&O: C, 84.90; and the two-phase system was separated. The ether layer was H, 11.32; mol wt, 424. Found: C , 84.63; H, 11.38; mol wt washed with water, 5% sodium bicarbonate, and water, and dried (mass spectrometry), 424. over anhydrous magnesium sulfate. Removal of solvent afforded A1z-Oleanene-30,30d~(XVI).-The aldehyde XIV (22 mg) was A-12-oleanen-28-ol ( 190 mg) which upon rcerystallization from converted to its ethandithioacetal and the latter, without chloroform-methanol furnished a microcrystalline material: purification, was desulfurized to A1z-01eanene-30,30-d~ (XVI) by mp 92-96", [ a ] 2 8 . 8 D$79.68" (c 0.74). Anal. Calcd for CaoHNO: using deuterium-containing Raney nickello as previously demol wt, 426. Found by mass spectrometry: mol wt, 426. scribed. Chromatography over l g of neutral alumina, activity A1z-Oleanen-28-al (V).-A mixture of 55 mg of IV, 12 mg of I , using n-hexane as eluent, furnished 8 mg of pure XVI: mp 156-158", after recrystallization from methanol-chloroform; pyridinium trifluoroacetate, and 74 mg of dicyclohexylcarbodiimide in 1 ml of dimethyl sulfoxide and 1 ml of ether was stirred mass spectrum: mle 410 (do), 7%; m/e 411 ( d l ) , 19%, mle 412 a t room temperature overnight.* The reaction mixture was ( d d , 62%; m/e 413 (&I, 12%. A1z-Oleanen-3-one (XVIII) .- Jones oxidation7 of j3-amyrin poured into water, extracted with ether, and the ethereal extract was washed with dilute hydrochloric acid, water, 5y0 sodium (XVII), mp 197-199', as previously described afforded j3bicarbonate, and water. Drying over anhydrous magnesium amyrone (XVIII) in quantitative yield, mp 176-178' after recrystallization from methanol-chloroform (lit." mp 178-180'). sulfate, followed by removal of solvent under reduced pressure, A12-01eanene-3,3-d2(=).--A mixture of 100 mg of j3-amyrone provided a froth, which was subjected to column chromatography, using 4.5 g of netural alumina, activity 11. Elution with n(XVIII), 100 mg of p-toluenesulfonylhydrazide, and 4 drops of hexane gave 22 mg of A1z-oleanen-28-a1 (V): mp 165-168', acetyl chloride in 6 ml of methanol w&s heated to reflux tempera3.69 p ture and dioxane was added dropwise until a homogeneous soluafter recrystallization from methanol-chloroform; ?: :A tion resulted. The latter was maintained a t reflux overnight, (w), 5.80 fi (9). Increasing the eluent polarity to 1:2 n-hexanecooled, poured into water, and the organic material was extracted benzene afforded 19 ing of starting material IV. Anal. Calcd with ether. The ethereal extract was washed with water, dilute for C ~ O H ~ mol O : wt, 424. Found by mass spectrometry: mol hydrochloric acid, water, 5% sodium bicarbonate, and water, wt, 424. and dried over anhydrous magnesium sulfate. Removal of A12-01eanen-28-alEthane Dithioacetal (VI) .-A solution of 59 mg of the aldehyde V, 0.5 ml of ethanedithiol, and 2 ml of boron solvent under diminished pressure afforded an oil which by thin trifluoride-ether complex in 8 ml of glacial acetic acid was stirred layer chromatographic analysis was shown to consist of the expected tosylhydrazone (XIX) and the starting ketone. This a t room temperature overnight. The resulting solution was then mixture was not separated but reduced directly with lithium poured into water, extracted with ether, and the ether extract was aluminum deuteride.l8 washed with dilute, aqueous sodium hydroxide (eight times) and To a stirred suspension of 150 mg of lithium aluminum deuwith water. After drying over anhydrous magnesium sulfate, the organic solvent was evaporated under diminished pressure to teride in dry dioxane (freshly distilled from sodium) was added leave an oil, which was chromatographed over 8 g of neutral dropwise a solution of the crude tosylhydrazone X I X in the same solvent. The resulting suspension was heated under reflux for 2 alumina, activity 11. The n-hexane eluate afforded 54 mg of hr, while being kept under an atmosphere of nitrogen. The A%leanen-28-a1 ethanedithioacetal (VI), which did not exhibit reaction mixture was cooled in an ice-water bath and deutercarbonyl absorption in the infrared spectrum. Recrystallization ium oxide was added cautiously dropwise. Excess anhydrous from methanol-chloroform produced crystals of mp 173.5175'. magnesium sulfate was added and the suspension was filtered. Anal. Calcd for C32HSZS2: C, 76.80; H, 10.40; mol wt, 500. The filter cake was washed well with ether. The filtrate was Found: C, 76.52; H, 10.38; mol wt (mass spectrometry), 500. washed with water, 5% sodium bicarbonate (twice), and water, 4lZ-01eanene-28,28-d2(VII).-A mixture of 30 mg of the dithioacetal VI and ca. 1 g of deuterium-containing Raney nickello dried over anhydrous magnesium sulfate, and evaporated to dryness. A thin layer chromatogram indicated that the residue m 10 ml of 0-deuteriomethanol was maintained at reflux overwas a mixture of alcohol XVII (containing a 3c~deuterium atom) night while under nitrogen. The catalyst was filtered by passing the reaction mixture over activity I11 alumina and the solvent and hydrocarbon compounds, by comparison with thin layer chromatographic mobilities of authentic material. Chromatogevaporated under reduced pressure to yield a colorless powder. raphy over 1 g of neutral alumina, activity 11, using n-hexane aa The latter was taken up in ether and the ethereal solution was eluent afforded the nonpolar product, which by thin layer washed with water and dried over anhydrous magnesium sulfate. chromatographic analysis, using a 5% silver nitrate impregnated Removal of solvent under vacuum afforded a glass which was chromatographed over 2 g of neutral alumina, activity 11, using silica gel G plate,41 was shown to consist of X X (by direct comn-hexane as eluent, to provide 20 mg of A12-oleanene-28,28-d~: parison with authentic nondeuterated material) and a less polar compound (probably Az,l%leadiene and ring-contracted matemp 162-164' after recrystallization from methanol-chloroform; rialat). The desired A1a-oleanene-3,3-d~ (12 mg) was isolated by mass spectrum: mle 410 (do), 7y0; m/e 411 (dl), %yo; m / e 412 preparative thin layer chromatography using a 5% silver nitrate ( d d , 63%; mle 413 ( t i s ) , 6%. impregnated silica gel H ~hromatoplate:4~mp 156-158' after A1z-01eanen-30-ol(XIII) .-In practice it was found easiest not recrystallization from methanol-chloroform; mass spectrum: mle to isolate the carboxylic acid intermediates in the transformation I X + x + X I 4 XI:[ -+ XIII. 410 (do), 9%; m/e 411 ( d l ) , 42%; mle 412 ( d d , 49%. A1z-Oleanen-11-one (XXII).-To a boiling solution of 300 mg A solution of 400 nng of methyl deoxyglycyrrhetate ( IX)lZin of A1%leanene (XXI)43 in 18 ml of glacial acetic acid was added 70 ml of 10% potassium hydroxide in methanol was heated under dropwise a solution of 425 mg of chromium trioxide in 15 ml of reflux for 22 hr. The cooled solution was poured into water, acidified with 18 ml of concentrated hydrochloric acid, and ex85% acetic acid14 and the resulting solution stirred vigorously and heated under reflux for 2 hr. The cooled solution was poured tracted with ether. The ethereal extracts were washed thrice into water, extracted with ether, and the ethereal extract ww with water and dried over anhydrous magnesium sulfate. Evapwashed with water, dilute sodium hydroxide solution, and water. oration of the solvent afforded a froth (X) which was converted to the desired alcohol XI11 by chromic acid oxidation7 to the After drying over anhydrous magnesium sulfate, the solvent was keto acid X I followed by Huang-Minlon reactions to the acid evaporated under diminished pressure to provide a colorless solid. The crude product was chromatographed over 30 g of silica gel XI1 and subsequent lithium aluminum hydride reduction as previously described. Recrystallization from methanol-chloro(Grace-Davison Chemical Co., Baltimore, Md). n-Hexane form afforded pure A1~'-oleanen-3O-olXIII: mp 173-175" (kagelution furnished 30 mg of starting material (XXI), while increasing the eluent polarity to 4:l n-hexane-benzene gave a mp 178'). small amount of by-product (not characterized). Further elution A12-01eanen-30-al (XIV).--Oxidation of 66 mg of XI11 to the aldehyde XIV was performed according to the procedure of (38) Prepared by adding 2 ml of trifluoroacetic acid dropwise to a stirred solution of 4.5 ml of pyridine in 15 ml of ether. The white solid whioh results is oolleoted by filtration, washed with ether and used immediately. (39) 0.Jeger and W. Hofer, Helv. Chim. Acta, 81, 157 (1948).
(40) G . Brownlie, M . B. E. Fayes, F. 5. Spring, R. Stevemn. and W. 5. Strachan, J . Chem. Soc., 1377 (1956). (41) A. 9. Gupta and 9. Dev, J . Chromatog., 14, 189 (1963). (42) R. H.Shapiro, Tefrahedron Letters, in press. (43) E. J. Corey and J. J. Uraprung, J . A m . Chem. Soo., 78, 5041 (1956).
JUNE1966
PENTACYCLIC TRITERPENE HYDROCARBONS
with 2 : l n-hexane-benzene afforded 210 mg of XXII. Two recrystallizations from methanol-chloroform gave pure A12oleanen-ll-one: mp 216216'; ' : :A 250 mp ( e 12,600); 6.08 p (s), 6.17 p (m); ORD (c 0.10, in dioxane): [+IM9 +957', +3830', [+Isso +3447', [+1369 +4404', [+I362 +4021' [@I362 +5936', [+]3.12 +7758', [+I254 $42,130". Anal. Calcd for CaoHasO: mol wt, 424. Found by mass spectrometry: mol wt, 424. A12-Oleanene-11,l I-dz (XXIII).-A suspension of 30 mg of platinum oxide in 0-deuterioacetic acidu was prereduced with deuterium gas and a solution of 18 mg of the a,p-unsaturated ketone XXII in 0-deuterioacetic acid was added. After stirring the suspension in the presence of deuterium gas for 5 days, the catalyst was filtered and the filtrate was poured into water, extracted with ether, and the ethereal solution was washed with 5y0 NaHCOa (twice) and water (three times). The dried solution was evaporated to dryness, leaving 15 mg of a colorless solid shown to consist of a mixture of starting material and desired product by thin layer chromatographic analysis. Chromatography of the crude reaction product over 1 g of neutral alumina, activity I, and methanol-ether recrystallization of the material obtained from the first two n-hexane fractions afforded AIL oleanene-ll,ll-dz: mp 158-160'. The mass spectrum of this substance exhibited e. weak molecular ion region, precluding accurate determination of isotopic purity from the M+ region. The isotopic distribution was calculated from the m/e 220 area (retro-Diels-Alder fragment) and was found to be 3% do, 17% dl, and 80% dz. 18a-A12-Oleanen-ll-one (XXIV).-A solution of 172 mg of A12oleanen-ll-one (XXII) in 25 ml of 15% potassium hydroxide in absolute ethanol was heated under reflux under nitrogen for 52 hr.l4 The cooled solution was poured into water, extracted with ether, and the ethereal extract was washed with water (twice), dilute hydrochloric acid, and water. The dried solution was evaporated to dryness under vacuum, leaving a yellow amorphous material. The crude product was subjected to a careful gradient elution chronnatography over 23 g of silica gel (GraceDavison Chemical Co., Baltimore, Md.). Elution with 10% n-hexane-benzene gave 27 mg of the desired product XXIV (fractions 38-55) and 54 mg of a mixture of 18a and 18p isomers (fractions 56-88). Increasing the eIuent polarity to 20y0 hexane-benzene provided an additional 51 mg of a mixture of the isomeric a,@-unsaturated ketones. The impure fractions were combined and the chromatographic procedure repeated to afford an additional 20 mg of desired product XXIV. Pure 18~~-A~~-oleanen-ll-onc~ was recrystallized from methanol245 mp ( e 10,920); ": : :A 6.02 chloroform: mp 181.5-182';" : :A (s), 6.12 p (m); ORD (c 0.11, in dioxane), [+]s89 +391', [+]a76 -2266O, [+I365 -1016', [+]a60 -1407', [+I256 +22,158', [+I240 +8441'. 18a-A1z-Oleanen-l1-one-28,28-d2(XXVI).-Allylic oxidation of VI1 (isotopic purity: 147, do, 27% d l , 51% dz, and 8% d3) to the enonc XXV and sub6,equent base equilibration to form XXVI was performed as previously described for the analogous sequence with nondeuterated compounds. The dp analog XXVI upon recrystallization from ethanol-chloroform exhibited mp 281.5283"; 6.02 (s), 6.12 p (m). Anal. Calcd for CaoHieDzO: D, 11.59. C, 84.51; H D, 11.74. Found: C, 84.33; H A11,1a(1a)-Oleadiene-28,28~z (XXVIII).-Lithium aluminum hydride reduction, as previously described, of XXVI afforded 18~-A12-oleanen-ll@-ol-28,28-d~ (XXVII),l* which was not isolated but directly subjected to the following reaction conditions. A suspension of 28 mg of the crude allylic alcohol XXVII and 20 mg of platinum oxide in glacial acetic acid was stirred in the presence of hydrogen gas for 24 hr. The catalyst was filtered and the solution was evaporated to dryness. The residue was chromatographed over 1 g of neutral alumina, activity 11, using nhexane as eluent to Flrovide 25 mg of colorless material, homogenom on a silver nitrate impregnateddl thin layer chromatogram. Recrystallization from methanol-chloroform afforded crystals melting a t 203-206'; Xfkt" 242 mp ( e 23,000), 250 mp ( e 25,800), D Z wt, : 410. 259 mp ( e 17,500). Anal. Calcd for C ~ O H ~ ~mol Found by mass spectrometry: mol wt, 410. 18c~-A~~-Oleanene-2!3,28-d~ (XXIX).-To a blue solution of ca. 20 mg of lithium metal in 5 ml of freshly distilled ethylamine was added a solution of 55 mg of the crude allylic alcohol XXVII in tetrahydrofuran% (freshly distilled from lithium aluminum hydride). The reaction was continued for 5 min while using a Dry Ice-isopropyl alcohol condenser to contain the ethylamine. Excess lithium was decomposed by dropwise addition of methanol a t
+
+
1955
0' and the resulting colorless mixture was warmed gently to remove the organic solvent. The residue was taken up in etherwater and the ethereal solution was washed with water, dilute hydrochloric acid, water, 5% sodium bicarbonate, and water. After drying over anhydrous magnesium sulfate, the solvent was evaporated under diminished pressure, leaving an oil which was chromatographed over 3 g of neutral alumina, activity I. Elution with n-hexane afforded 40 mg of 18a-A12-oleanene-28,28dz: mp 187.5-189.5' after recrystallization from methanolchloroform; mass spectrum: m/e 410 (do), 17%; m/e 411 (&), 28%; m/e 412 (dz), 50%; m/e 413 (da), 5%. A12-Ursen-28-ol (XXXIII).-Ursolic acid was converted to XXXIII in three steps as described above for the similar transformation in the A1koleanene series. However, in the present case the primary aicohol XXXIII could not be induced to crystallize. The amorphous material which was obtained exhibited a melting point range of 77-89', [ ( Y ] ~ D+72.64' ( c 1.0) Anal. Calcd for CaoH5oO: mol wt, 426. Found by mass spectrometry: mol wt, 426. Ala-Ursen-28-al (XXXIV).-Moffat oxidation,# as previously described, of 274 mg of A12-ursen-28-ol(XXXIII) resulted in the isolation of 120 mg of XXXIV: mp 96-102' after recrystallization from aqueous methanol (lit.44mp 172-174'); A:,"*' 5.80 (s), 3.69 p (w). Anal. Calcd for C30H480: mol wt, 424. Found by mass spectrometry: mol wt, 424. A12-Ursen-28-al Ethane Dithioacetal (XXXV).-Prepared as described in the preparation of A12-oleanen-28-alethane dithiolacetal (VI). The crude product obtained from reaction of 105 mg of aldehyde XXXIV, when subjected to chromatography over 20 g of neutral alumina, activity 11, using n-hexane as eluent afforded 90 mg of XXXV, mp 95-98' after recrystallization from methanol-chloroform. Anal. Calcd for CazH&2: mol wt, 500. Found by mass spectrometry: mol wt, 500. A12-Ursene-28,28-dz(XXXVI).-Desulfurization of 57 mg of XXXV with deuterium-containing Raney nickel in O-deuteriomethanol as previously described provided 23 mg of A'*-ursene28,28-dz (XXXVI). Two recrystallizations from methanolether gave crystals melting a t 105-109'; mass spectrum: m/e 410 (do), 7%; m/e 411 ( d l ) , 23%; m/e 412 (dz), 62%; m/e ( d 3 L 8%. A12-Ursen-3p-ol Benzoate-12,27-d~ (XLII) .-A solution of 195 mg of phyllanthyl benzoate22 in 7 ml of alcohol-free chloroform was placed in a 25-ml three-necked flask equipped with a gas inlet tube and connected to a water aspirator.Z1 Deuterium chloride gas was generated by the dropwise addition of distilled phosphorus trichloride to stirred deuterium oxide, maintained a t room temperature in a 100-ml three-necked flask which was connected by tygon tubing to a calcium chloride tube and the latter joined to the smaller three-necked flask by tygon tubing. The reaction vessel was partially evacuated and gaseous deuterium chloride admitted. This evacuation and deuterium chloride addition was repeated two additional times, the reaction vessel was closed, and the solution was stirred a t room temperature overnight. Evaporation of solvent afforded a quantitative yield of deuterated a-amyrin benzoate (XLII), mp 196.5-199' after recrystallization from methanol-chloroform. For isotopic purity, see compound XLV. A12-ursen-3p-ol-12,27-d2 (XLIII).-Lithium aluminum hydride reduction, as mentioned earlier, converted 180 mg of the benzoate (XLII) to 140 mg of the free 3p alcohol (XLIII). This product, upon recrystallization from methanol, exhibited a melting point of 186-188'. A12-Ursen-3-one-12,27-dz (XLIV).-The usual Jones oxidation' of 110 mg of XLIII gave 100 mg of the deuterated a-amyrone (XLIV), mp 124-125' after a single recrystallization from methanol. A12-Ursene-12,27-dz (XLV).-The previously described HuangMinlon reduction of 70 mg of XLIV afforded 61 mg of the isotopically labeled hydrocarbon XLV: mp 111-117' after recrystallization from methanol-chloroform; mass spectrum: m/e 410 (do), 20%; m/e 411 (dl), 35%; m/e 412 (dz), 42%; m/e 413 (d3), 3%. A12-Ursene (XLI) .-Conversion of phyllanthyl benzoate (XXXVII)zl? to phyllanthane (XL) was accomplished by the lithium aluminum hydride reduction-Jones oxidation'-HuangMinlon reductions route discussed above. Opening of the cyclopropane ring was affected according to literature directions*' by (44) J. Polonsky, Compt. Rend., 133, 671 (1951).
1956
MATSUMOTO AND HARADA
heating a solution of 40 mg of phyllanthane in 10 ml of acetic acid and 1ml of concentrated hydrochloric acid a t reflux temperature. The cooled reaction was poured into water, extracted with ether, and the ethereal extract was washed with water, 5% sodium bicarbonate, and water. After drying over anhydrous
VOL.31
magnesium sulfate, the solvent was evaporated under reduced pressure to afford an oil which was chromatographed over 1 g of neutral alumina, activity 11. Elution with n-hexane gave 15 mg of A'kursene (XLI), mp 100-105" after recrystallization from methanol-chloroform.
Stereoselective Syntheses of Optically Active Amino Acids from Menthyl Esters of a-Keto Acids' KAZUO MATSUMOTO AND KAORU HARADA Institute) of Molecular Evolution and Department of Chemistry, University of Miami, Coral Gables, Florida Received January 6, 1966 Menthyl asters of pyruvic acid, a-ketobutyric acid, and phenylglyoxylic acid were converted to their oximes and Schiff bases of benzylamine. These were hydrogenated catalytically by the use of palladium on charcoal and palladium hydroxide on charcoal. Opticallv active D-alanine (optical vield 16-25%,), D-a-aminobutvric acid ?8-21%), and D-phenylglycine (44-49%) weie obtained. Possiblesteric Eourses of the-reactions are discussed.
I-Menthol has been used as an optically active moiety in many types of stereochemical studies.2 I n previous studies the menthyl esters of various a-amino acids have been synthesized by the azeotropic method3 and from a-amino acid N-carboxyanhydrides. I n this study, the N-hydroxyimino (oxime) and Nbenzylimino (benzylamine Schiff base) derivatives of menthyl pyruvate, menthyl a-ketobutyrate, and menthyl phenylglyoxylate were synthesized. These were hydrogenated and hydrogenolyzed by the use of palladium on charcoal (catalyst A) or palladium hydroxide on charcoal5 (catalyst B) to yield the menthyl esters of alanine, a-amino-n- butyric acid, and of phenylglycine. These asymmetrically synthesized menthyl esters of amino acids were saponified by alkali in aqueous alcohol.6 The liberated, free amino acids were isolated and their optical activities measured to determine the optical yield. However, the isolation and recrystallization procedure resulted in fractionation of the optically active amino acids The specific rotation of amino acids decreased upon recrystallization and finally the value reached zero aSter several purification procedures, To avoid the fractionation and to determine the accurate optical purity of the synthesized amino acids, a part of the hydrolyzed reaction mixture was directly treated with l-fluoro-2,4-dinitrobenzeneto yield dinitrophenylamino acids.' The resulting DNP-amino acids were isolated chromatographically by the use of a Celite column treated with pH 7 phosphate-citrate The DNP-amino acids thus obtained were analytically pure without further purification. An (1) Sterically Controlled Syntheses of Optically Active Organic Compound. I. Contribution No. 059 of the Institute of Molecular Evolution, University of Miami. (2) A. McKenzie, J. Chem. Soc., 85, 1249 (1904);A. McKenzie and H. B. P. Humphries, ibid., 95, 1105 (1909); A. McKenzie and I. A. Smith, Ber., 68, 899 (1925); V. Prelog, Helu. Chim. Acta, 36, 308 (1953); H. M. Walborskey, et d., J . A m . Chem. Soc., 81, 1514 (1959); 88,2517 (1961); 1 ' . Inoue, et al., ibid., 89,5255 (1960). (3) K. Harada and T. Hayakawa, Bull. Chem. Soc. Japan, 37, 191 (1964). (4) T.Hayakawa and K. Hrtrada, ibid., 88, 1354 (1965). (5) R. G. Hiskey and R . C. Northrop, J . A m . Chem. Soc., 85, 4798 (1961). (6) DL-Amino acid menthyl esters were fractionated easily during the isolation and recryatallization procedure;*" it was found difficult to estimate the optical yield by isolation of the menthyl esters without fractionation. (7) F. Sanger, Biochen. J . , 39, 507 (1945); F. C. Green and C. M. Kay, Anal. Chem., 14, 726 (1952); K. R. Rao aud H. A. Sober, J . A m . Chem. Soc., 7 6 , 1328 (1954). (8) J C. Perrone, Nature, 167, 513 (1951); A. Courts, Biochem. J . , 68, 70 (1954
advantage of the DNP method was to avoid fractionation completely during the isolation and purification procedures. Table I shows the summarized results which were obtained by the catalytic hydrogenation procedure. Partially optically active (849%) D-amino acids were obtained. However, the optical activities of the amino acids prepared by the use of palladium on charcoal (catalyst A) from the Schiff base of pyruvate and of a-ketobutyrate were found to be zero. The optical activity of a-aminobutyric acid prepared by the use of palladium hydroxide on charcoal (catalyst B) was also zero; however, the DNP-a-aminobutyric acid showed optical activity, [cY]*~D -7.3". The latter case can be explained by the fractionation of the product during the isolation procedure. The results show that the hydrogenation reaction of hydroxyimino derivatives (oxime) gave higher optical activity than that of benzylimino derivatives (Schiff base). Menthyl phenylglyoxylate did not form the Schiff base with benzylamine under the azeotropic distillation method which was employed for the other keto esters. To check the racemization of amino acids during the saponification by alkali, the authentic menthyl esters of D-alanine, D-a-aminobutyric acid, and Dphenylglycine3r4were hydrolyzed by the same procedure employed in the hydrolysis of the menthyl esters synthesized in this study. Optical rotations were measured as DNP-amino acid which was separated by column chromatography. Racemization of D-alanine and D-CYaminobutyric acid was slight (4 and 3%) respectively) ; however, D-phenylglycine lost 77% of its activity during the saponification procedure. The optical purities listed in Table I are corrected by use of the values of standard racemization. The steric course of the synthesis could be explained in a way similar to the rules proposed by Cramg and Prelog'o as is shown in Scheme I. The most stable conformation might be structure I since C=O and C=N groups repel each other because of their electric dipoles. The menthyl residue is considered to take a conformation as was proposed by PreloglO (Scheme I). The molecules would be absorbed with the less bulky side on a catalyst, and the hydrogen atoms would attack (9) D.J. Cram and F. A. Abd Elhafez, J . A m . Chem. Soc.. 74,5828 (1952). (10) V. Prelog, Helu. Chim. Acta, 36, 308 (1953).
